Memory test systems typically test memory modules such as CMOS dynamic random access memories (DRAM) for shorted or open data lines and shorted or open address lines and shorted or open control lines. Before such tests may be conducted, however, a contact test is required to ensure that the test system is in contact with the pins of the DRAM, as well as whether the pins are shorted together or open. All known testers test with power applied to the memory I/O pins and a current of the order of 200 .mu.amps to avoid damaging the memory module under test.
A recognized reference on contact tests is the text "Testing Semiconductor Memories, Theory and Practice", by A. J. van de Goor, John Wiley & Sons, copyrighted 1991, reprinted 1996. In the text, pages 295-297, van de Goor proposes a test circuit where all input and output pins of a memory chip are connected to the Vcc power line and the Vss ground line by way of protection diodes, which are said to ensure that the input and output pins cannot assume a voltage level on one diode voltage drop below Vss or one diode voltage drop above Vcc. A test algorithm for the contact test also is proposed which consists of the following steps:
1. Set all pins to 0 volts.
2. Force a forward biasing current through the diodes in the range of 100 .mu.amps to 250 .mu.amps.
3. Measure the voltage Vpin resulting from the forward biasing current, and
if .vertline.Vpin.vertline.&lt;0.1 V, a short is assumed; PA1 if 0.1V.ltoreq..vertline.Vpin.vertline..ltoreq.1.5 V, contact is assumed; and PA1 if .vertline.Vpin.vertline.&gt;1.5 V, an open is assumed.
In the above example, the tester cannot distinguish between a short to Vcc or ground, and a short to another pin.
U.S. Pat. No. 5,072,175 to Marek discloses a test method and system for identifying only pin contacts and opens. Pins shorts to a power supply terminal or to ground, and pin to pin shorts are not identified. Further, modification to the component under test is required to add diodes and a test terminal, whether component is a chip or a module.